Recently, rechargeable batteries, i.e., secondary batteries, have been widely used to provide electrical power for driving battery powered electronic appliances such as, by way of example, portable radio cassette players, portable computers, camcorders, cellular telephones and other devices. Alkaline batteries such as nickel cadmium (Ni--Cd) or nickel metal hydride (Ni--MH) batteries have been generally used as the secondary battery. Recently, lithium ion (Li-ion) batteries with an organic electrolytic cell have gained popularity in high-end portable electronic devices because they exhibit high energy density, low temperature characteristics, and stable storage capability.
Rechargeable batteries require an electronic charger for recharging depleted batteries. A charger should include an internal charger circuit incorporated into the battery powered appliance. A charger will begin charging the battery whenever the device is powered by alternating current (i.e., AC) power. External battery chargers accepting one or more batteries to be charged, are equipped with an independent power supply and connectors.
Although rechargeable batteries have various types of battery chemistry, battery pack voltage, and battery pack capacity, there have been few methods of charging the batteries adopted in battery chargers. Generally, the charging method is either a constant voltage charging process or a constant current charging process. Constant voltage charging applies a constant voltage that is higher in amplitude than the nominal voltage of the battery across the terminals of a battery. Constant voltage charging process is typically used for charging a backup battery where frequent charging and discharging is not occurring. The charging voltage is continuously applied to the battery. On the other hand, the constant current charging process applies a constant current to the battery irrespective of any increase in the voltage across the terminals occurring as the charging progresses. Constant current charging is useful for rapidly charging a battery. Constant current charging however, requires a time limit in order to avoid damage of the battery due to overcharging.
Except those used in the portable radio cassette players, most battery chargers use a constant voltage charging process because it is simple in construction and is relatively inexpensive. This charging process requires relative long time, approximately about ten hours. Thus, constant voltage charging processes can be ineffective for portable use of the appliance powered by batteries.
In an effort to provide a more rapid charging process with battery protection, a charging mode convertible type charger has been developed. This charger performs charging operations in the both constant current mode and in the constant voltage mode. This type of charger starts charging the battery in a constant current mode when the battery is discharged in order to provide a fast charging operation, and automatically converts to a constant voltage mode at a predetermined charging level in order to complete a typical charging operation. Thus, rapid charging is provided while avoiding damage to the battery due by allowing the battery to remain connected to the charger for an excessively long time.
Some circuit designs such as the Apparatus For Controlling Charging Of A Storage Battery of Yeong J. Joo, U.S. Pat. No. 5,175,485 seek to initially apply a constant current, and then a constant voltage when the voltage across the terminals of a battery reach a desired value. Recent circuit designs such as the Rechargeable Battery Charging Method of M. Tamai, U.S. Pat. No. 5,442,274, have used hysteresis charging to a set value, followed by constant voltage charging. Other recent efforts in the art include the Method For Charging Secondary Battery And Charger Used Therefor by T. Nagai, et alii, U.S. Pat. No. 5,576,608 and the Method For Charging A Secondary Battery And Charger Used Therefor Using Constant Current And Constant Voltage by T. Nagai, et alii, U.S. Pat. No. 5,637,981, for converting after detection of various characteristics from a constant current to a constant voltage that is equal to the fully charged voltage.
In a constant current charging process, rapid and full charging is possible within a time limit during which a constant current is applied to the battery. In the mode convertible type charging process, initial charging is performed in the constant current mode for a predetermined time period, and then the charging operation is switched to the constant voltage mode, so that a full charge of the battery is achieved with the charging voltage being constantly maintained.
Ni--Cd and Ni--MH batteries can require the constant current charging mode, while Li-ion batteries require both the constant charging mode and the constant voltage charging mode. In the constant current charging mode, the end point of time at which the battery must cease its charging is determined by detecting an increase in the terminal voltage of the battery when the battery is fully charged. There is some difficulty in detecting the exact charge end point, because the full charge time varies with the residual capacity of the battery.
In a specific rapid charger, the charge time is determined by detecting voltage drop across the battery terminals because some amount of current is discharged from the battery when charging begins. The charge time is determined in accordance with an internal charging program. This method is difficult because the full charging of a battery can not be assured during the preset charging time due to the fact that the preset charging time does not reflect aging of the battery. Moreover, unless the battery to be charged is made by the manufacturer of the charger, the full charge time necessary for the battery can not be uniformly provided to the battery. Consequently, the terminal voltage of the battery being charged varies with the residual capacity and degree of aging of the battery. The conventional battery charger has endeavored to provide a compromise between rapid full charging and protection of the batteries.
A conventional charger for charging a Li-ion battery by using both a constant voltage charging process and a constant voltage charging process supplies a charging current to the battery by controlling the current or voltages between the output of a switching regulator and the battery being charged. This mode convertible type of battery charger unnecessarily consumes power because it always performs a constant voltage charging operation. When a Li-ion battery is being charged, the constant voltage charging mode is only necessary for supplemental charging subsequent to the constant current charging. Moreover, the constant voltage control circuit of the charger is apt to be damaged by the occurrence of static, noise or surges when the battery pack is either mounted on or detached from the appliance.